Surfactants suspension particle adhesion

Figure 4.19 Illustrating detergent action, (a) Surface covered with greasy dirt, (b) Detergent is added to the solution. The surfactant molecules reduce adhesion of dirt to the surface when they are deposited with their hydrophobic tails on the solid surface or dirt particle. The dirt particles are thus more readily removed by mechanical action, (c) Dirt particles are held as a suspension by an adsorbed layer of surfactant. [Reproduce with permission of the estate of Irving Geis]...

This topic is dealt with in great detail by Uno and Tanaka [45-47]. Apart from the energies of repulsion and attraction due to adsorption of surfactants, adhesion may result from crystallization under the special conditions that obtain when particles are left behind after drainage of the bulk phase. Particles will be trapped by the surface tension of the liquid film (Fig. 9.18). As the particle dissolves there is supersaturation followed by deposition of the solid around the particle. According to Uno and Tanaka the solubility of the suspension particle increases at the particle-wall contact point due to the pressure exerted on the particle by the surface tension forces. Following deposition, evaporation of the continuous phase occurs as depicted in Fig. 9.18 [46]. Deposition due to cooling, to medium evaporation and to the adhesiveness of additives in the formulation can occur. The effect of polysorbate 80, benzethonium chloride, some anionic surfactants and poloxamers on the adhesion of chloramphenicol suspensions has... 

A final important class of composite materials is the composite hquids. Composite liquids are highly stmctured fluids based either on particles or droplets in suspension, surfactants, liquid ciystalhne phases, or other macromolecules. A number of composite liquids are essential to the needs of modem industiy and society because they exhibit properties important to special end uses. Examples include lubricants, hydraulic traction fluids, cutting fluids, and oil-drilling muds. Paints, coatings, and adhesives may also be composite liquids. Indeed, composite hquids are valuable in any case where a well-designed liquid state is absolutely essential for proper delivery and action. 

If surfactant is added to a suspension polymerization system, a number of phenomena may occur. If the surfactant is added in small amounts (below the critical micelle concentration or CMC), the reduction in interfacial tension between the organic and aqueous phases will result in smaller monomer droplets, but it has hardly any other effect. If surfactant is added above the CMC, and an oil-soluble initiator is used, the process is commonly termed a microsuspension polymerization. Due to the reduced interfacial tension, the droplet diameter (and hence bead diameter) is reduced to approximately 10-40 pm. Little polymerization takes place in the aqueous phase or in particles generated from surfactant micelles because of the hydrophobic nature of the initiator. However, some smaller particles initiated from surfactant micelles may be found. The kinetics are still essentially those of a bulk free radical polymerization. Microsuspension polymerization is used to produce pressure-sensitive adhesives for repositionable notes. 

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

The surfactant concentration giving the maximum adhesion will depend on the nature of the surfactant and surface in the case of adhesion of particles present in a suspension, this surfactant concentration will also depend on the suspension concentration. 

The adhesion of a layer of particles formed on a glass surface by settling from a suspension varies with the concentration of cationic surfactant in the following manner ... 

The adhesion of suspended particles to the vessel walls will depend on the number of cycles of charging and discharging slurry from the vessel. Experiments have shown [191] that with a suspension concentration of 15%, the maximum number of adherent particles is reached after the 24th cycle, amounting to 29 mg/cm of vessel surface. The number of adherent particles can be changed by adding the surfactant to the suspension, specifically a cationic surfactant such as... 

In hard-surface cleaning, solid particles deposit primarily from air suspensions and embedded in greasy soils, such as silica and metal oxides, will have their wetting characteristics modified by surfactants. Through surfactant adsorption at the water-solid particle interface, the interfacial tension will be reduced, and so will be the adhesion forces binding particles together. Efficient surfactants are anionic surfactants that, when adsorbed, make the surface of the solid particles more negative and induce electrostatic repulsion between adjacent particles [60]. 

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